The backing for the conventional plasters for percutaneous absorption may be polyvinyl chloride film; olefin films such as polyethylene and polypropylene; polyurethane film; polyester/polyethylene-vinyl acetate copolymer (EVA) laminated film; EVA/non-woven fabric (rayon) laminated film or non-woven fabric of polyester/polyester laminated film, and these are very thin.
These backings except for polyester/EVA laminated film are used to produce the plaster for the active ingredient having systemic effect such as isosorbide dinitarate and amyl nitrite which possess the characteristic of sublimation and fugacity. However, there are same cases that the active ingredient in the adhesive base material is absorbed by the backing, and diffused from the adhesive base material. Therefore, the plasters have to be stored in sealed condition. However, during the storage, the active ingredient contained in the adhesive base material may decrease by diffusing and absorbed to the container. As a result, the stable drug releasing from the adhesive layer may not be obtained.
On the other hand, to prevent diffusion and absorption of the active ingredient to the backing, the polyester/EVA laminated film can be used to the backing of the plasters. However, because the close, adhesion to the skin is expected, the plaster must be laminated in thin layered composition preferably having thickness from 15 to 40 .mu.m. Therefore, the plaster using the polyester/EVA laminated film as backing has same problems of the curl caused by static electricity and internal stress when applied to the skin.
To overcome these disadvantages, it has been proposed to use the backing which is laminated with polyester film and non-woven fabric of polyester (Japanese Patent Publication No. Hei 5-309128). Nevertheless, even in the case of using the proposed backing, there are still same disadvantages. For example, in the case of containing the oily absorption accelerant, such as oleic acid, isopropyl myristate or crotamiton in the adhesive base material, these oily substance may be absorbed to the non-woven fabric of polyester. Therefore, the releasing capability of the active ingredient from the adhesive base material decreases as the time passes by. Additionally, the plaster may be torn when the polyester film is thin (i.e. less than 1.5 .mu.m), and the unit weight of non-woven fabric of polyester is less than 6 g/m.sup.2. As a result, the backing for the plaster satisfied all expectations has not been available.